It is not your business whether or not you neighbor is getting paid SSDI or what his diagnosed disabilities might be. It is not your business whether or not ANY one person is eligible including your spouse, your parent, your brother, your adult child, your best friend, your worst enemy, etc.
If you have reason to believe he is scamming the government, you can call the SSA OIG hotline and provide them with enough verifiable information like his full name (with correct spelling) and his date of birth so SSA might be able to figure out his SSN. Then, you need to provide what evidence you have that he is scamming the government. Unless you are a doctor, you cannot make a medical finding of whether or not he is disabled.
If he is bragging that he is scamming disability, you can probably get him to tell you how he's doing it (could pretend you want to also) and then you may have something concrete that could be investigated. But if you don't provide SSA with something more than you know he is scamming, there is little SSA can do.